The present invention relates to solid state light fixtures and, more particularly, to solid state light fixtures that are suitable for high temperature operation that emit white light at high luminous efficiency while maintaining good color rendering properties.
A wide variety of light emitting devices are known in the art including, for example, incandescent light bulbs, fluorescent light bulbs and so-called “solid state” lighting devices that use light emitting diodes (“LEDs”) as the light source. LEDs generally include a series of semiconductor layers that may be epitaxially grown on a substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with corresponding holes and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
Most LEDs are nearly monochromatic light sources that appear to emit light having a single color. Thus, the spectral power distribution of the light emitted by most LEDs is tightly centered about a “peak” wavelength, which is the single wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photo-detector. The “width” of the spectral power distribution of most LEDs is between about 10 nm and 30 nm, where the width is measured at half the maximum illumination on each side of the emission spectrum (this width is referred to as the full-width-half-maximum or “FWHM” width).
In order to use LEDs to generate white light, LED-based light emitting devices have been provided that include several LEDs that each emit a light of a different color. The different colored light emitted by the LEDs combine to produce white light. For example, by simultaneously energizing red, green and blue LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by surrounding a single blue LED with one or more phosphors that convert some of the light emitted by the LED to light of one or more other colors. The combination of the light emitted by the single-color LED that is not converted by the phosphors and the light of other colors that is emitted by the phosphors may produce a white or near-white light.
As one example, a white light emitting LED package may be formed by coating a gallium nitride-based blue LED (i.e., an LED that emits light having a peak wavelength in the blue color range as defined herein) with a “yellow” phosphor (i.e., a phosphor that emits light having a peak wavelength in the yellow color range) such as a cerium-doped yttrium aluminum garnet phosphor, which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce. The blue LED emits light having a peak wavelength of, for example, about 460 nm. Some of blue light emitted by the LED passes between and/or through the YAG:Ce phosphor particles without being converted, while other of the blue light emitted by the LED is absorbed by the YAG:Ce phosphor, which becomes excited and emits yellow fluorescence with a peak wavelength of about 550 nm (i.e., the blue light is converted to yellow light). The combination of blue light and yellow light that is emitted by the LED package may appear white to an observer. Such light is typically perceived as being cool white in color, as it is primarily comprises light on the lower half (shorter wavelength side) of the visible emission spectrum. To make the emitted white light appear more “warm” and/or exhibit better color rendering properties, red phosphors such as Eu2+ doped CaAlSiN3 based phosphor particles may be added to the coating applied to the blue LED.
In general, phosphors absorb light having first wavelengths and re-emit light having second wavelengths that are different (typically longer) than the first wavelengths. For example, “down-conversion” phosphors may absorb light having shorter wavelengths and re-emit light having longer wavelengths. It will be understood that the term “phosphor” is used broadly herein to encompass not only materials that have traditionally been referred to as phosphorescent, but also other luminophoric materials such as, for example, quantum dots, that absorb light at one wavelength and re-emit light at a different wavelength in the visible spectrum.
Typically, particles of a phosphor are mixed into a binder material such as, for example, an epoxy-based or silicone-based curable resin, and are then coated, sprayed or poured onto an LED or another surface of a light fixture. Herein, such mixtures are referred to as a “recipient luminophoric medium.” A recipient luminophoric medium may include one layer or the like in which one or more phosphors are mixed, multiple stacked layers, each of which may include one or more of the same or different phosphors, and/or multiple spaced apart layers, each of which may include the same or different phosphors.